26SI Series Alternator – Normal Operation

The alternator is a brushless, heavy-duty integral charging system. The alternator has a built-in diode rectifier and a voltage regulator. The system produces DC current for electrical systems.

The solid state integrated circuit voltage regulator that is built into the alternator limits the system voltage by switching the ground circuit for the field coil on and off. This is done rapidly in order to control the current that is in the field coil. When the ground circuit for the alternator field is turned ON by the voltage regulator, the field current passes from the diode trio and through the stationary field coil. Nominal regulated voltages of 13.5 to 14.5 volts are available for 12 volt systems. The nominal regulated voltage for the 24 volt system is between 27 and 29 volts.

After the engine is started and rpm rises, the excitation circuit is turned on all the time, and generated voltage rises rapidly. If the “I” terminal is not used, the initial field voltages at start-up are generated by residual magnetism. The residual magnetism can be lost. This results in no output. Loss of the residual magnetism can be caused by extended downtime or a severe shock to the alternator. Refer to Troubleshooting, “T6 Residual Magnetism Restoration”. As the speed increases and the output increases, the voltage that is available at the diode trio becomes sufficient to supply field current for normal operation. When the voltage at the “B” terminal exceeds the battery voltage current flows into the battery.

The 34SI model has an “I” terminal. The terminal CAN be used in order to supply excitation current. The current flows from a source that has a keyswitch through an indicator light. The indicator light provides a verification for alternator excitation and the light also provides a indication of faults. The “I” terminal must have an indicator or a resistor in series between the current source and the “I” terminal. This maintains the normal field current around 0.17 amperes. Once the alternator begins charging, the field current is supplied from the diode trio. Current stops flowing through the “I” terminal and the indicator lamp turns OFF.

The voltage regulator cycles the field current ON and OFF many times per second. This maintains the alternator output voltage at a preset level.

For 12 volt systems, an output rating of 105 to 110 amperes is standard. For 24 volt systems, output ratings of 60 to 100 amperes are available. The output ratings of a specific model are located in Specification, “Alternator”.

The output of the alternator must be connected to the positive terminal of the battery through the charging circuit for the machine. A ground path is also required. The ground path should run between the alternator ground terminal and the ground terminal for the battery.

While the system voltage is below the setting of the voltage regulator, the regulator turns ON the field current. This allows the alternator to produce the maximum output. When the voltage setting is reached, the regulator turns OFF the field current. When the field current is off, the magnetic field in the rotor collapses and the alternator output voltage begins to fall. The falling voltage causes the regulator to turn on the field current and the current rebuilds the magnetic field. This cycle continues rapidly. The cycle keeps the output and the system voltage very close to the voltage setting. The cycle will continue unless the electrical demands of the system cause the system voltage to fall below the voltage setting. If the system voltage falls below the voltage setting, the regulator will allow full field current to flow so that the alternator’s maximum output is realized. Maximum output is dependent on the alternator speed. At low speeds, the maximum output of the alternator is significantly reduced.

Illustration 1 g00651653

(1) Alternator Case Ground

(2) Internal Ground

(3) “I” Terminal

(4) “R” Terminal

(5) Output Terminal

Illustration 2 g00651654

(1) Output Terminal

(2) “R” Terminal

(3) Alternator Case Ground

External connections to the alternator are made to the terminals that are shown in illustration 1 and illustration 2. The standard output terminal is a male type. The connecting bolt is not insulated.

All of the electronic parts of the alternator are dipped in varnish in order to keep out the moisture and the dirt.